Goldschmidt2017 Abstract
The genesis of a “cycle” within UZc
of the Upper Zone of the Bushveld
Complex (South Africa)
BRITT, T.B.1, ROBERTS, R.J.2 AND HOLNESS,
M.B.3
1
University of Pretoria, Department of Geology,
([email protected])
2
University of Pretoria, Department of Geology,
([email protected])
3
University of Cambridge, Department of Earth Sciences,
([email protected])
The variability of P2O5 in the UZc of the Upper Zone of
the Bushveld Complex is unusual. Apatite in the UZc is not
present throughout the whole of the UZc stratigraphy, and
appears and disappears from the solidus over very short
intervals. P2O5 wt% increases from background values of
0.24 P2O5 wt% to 8.34 P2O5 wt% over less than a centimetre,
and the drop in P2O5 concentrations is equally as abrupt.
This study focuses on what is called a “cycle” within the
Upper Zone; Cycle V [1]. Tegner, et al. (2006) [1] attributed
these cycles to stratification of the Upper Zone magma
chamber by double diffusive convection and subsequent
forward crystallisation. This model addressed the unusual
reversals in Mg# and An%, as well as P 2O5 and V2O5, at the
boundary of each cycle throughout the Upper Zone.
This study seeks to determine the processes that resulted
in the lithology, the discontinuous mineralogy and variable
apatite behaviour found in Cycle V. Studies by Tollari, et al.
(2006) [2] and Toplis, et al. (1994) [2] have proven that SiO2
and P2O5 are intrinsically linked. Consequently, how did the
concentrations of SiO2-P2O5 shape the formation of the UZc
of the Upper Zone?
REFERENCES
[1] Tegner, C., Cawthorn, R., Kruger, F., 2006. Cyclicity in
the Main and Upper Zones of the Bushveld Complex, South
Africa: Crystallization from a Zoned Magma Sheet. Journal
of Petrology 47, 2257-2279. [2] Tollari, N., Toplis, M.J. &
Barnes, S.J. 2006. Predicting phosphate saturation in silicate
magmas: An experimental study of the effects of melt
composition and temperature. Geochimica et Cosmochimica
Acta, 70 (): 1518–1536. [3] Toplis, M.J. Libourel, G. &
Carroll, M.R. 1994. The role of phosphorus in crystallisation
processes of basalt: An experimental study. Geochimica et
Cosmochimica Acta. 58 (2): 797-810.